1. Field of the Invention
The present invention relates to a radiation image capturing apparatus for capturing a radiation image of a subject by applying a radiation emitted from a radiation source to the subject and detecting the radiation that has passed through the subject with a radiation detector.
2. Description of the Related Art
In the medical field, there have widely been used radiation image capturing apparatus, known as mammographic apparatus, which apply a radiation emitted from a radiation source to a breast of a subject and detect the radiation that has passed through the breast with a radiation detector.
One known radiation detector for use in the radiation image capturing apparatus includes a solid-state detector in a laminated structure comprising a matrix of charge collecting electrodes formed on an insulating substrate and a radiation conductor disposed on the charge collecting electrodes for generating electric charges depending on the radiation that is applied. The electric charges generated by the radiation conductor and representing radiation image information are collected by the charge collecting electrodes and temporarily stored in an electric storage unit. The collected electric charges are converted into an electric signal, which is output from the solid-state detector. Other known radiation detectors include a radiation detector comprising a charge-coupled device (CCD) and a radiation detector comprising a combination of amorphous silicon and a scintillator. Furthermore, a stimulable phosphor panel which, when exposed to an applied radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores part of the energy of the radiation, and, when subsequently exposed to applied stimulating light such as laser beam, visible light, or the like, emits stimulated light in proportion to the stored energy of the radiation, may also be used as a radiation detector.
In order to obtain a high-quality radiation image captured by a radiation detector, as shown in FIGS. 7 and 8 of the accompanying drawings, a grid 6 is disposed in front of a radiation detector 2 for preventing scattered rays of a radiation X that are generated in a subject 4 from entering the radiation detector 2, as disclosed in Japanese laid-open patent publication No. 2005-13344. As well known in the art, the grid 6 is a convergent grid comprising an alternate assembly of radiation-permeable members 8 made of aluminum or the like which pass the radiation X therethrough and radiation-impermeable members 10 made of a material including lead or the like, the radiation-impermeable members 10 being inclined parallel to the direction in which the radiation X is applied to the grid 6.
On the mammographic apparatus, it is customary to capture various radiation images of the breast in different directions, e.g., vertically, horizontally, and obliquely. Depending on the size of the breast, the breast may not be properly positioned in a prescribed position on the radiation detector 2. If the breast is not properly positioned in the desired position on the radiation detector 2, then the position of the radiation source 12 is changed into alignment with the position of the breast for appropriately irradiating the breast with the radiation X.
When the position of the radiation source 12 is changed, however, since the direction in which the radiation X is applied and the direction in which the radiation-impermeable members 10 of the grid 6 are inclined are brought out of alignment with each other, part of the radiation X may possibly be vignetted by the radiation-impermeable members 10.
In recent years, efforts have been made to perform tomosynthesis and stereoscopic imaging using mammographic apparatus. According to these imaging processes, the radiation source 12 is turned around the breast 4 in the directions indicated by the arrow α as shown in FIG. 7 to acquire a three-dimensional image or a desired sectional image of the breast 4. As the radiation source 12 is turned around the breast 4, the radiation X emitted from the radiation source 12 falls upon the grid 6 in constantly changing directions. Therefore, the radiation X is partly vignetted by the radiation-impermeable members 10 during the imaging process.
In order to avoid the vignetting, the grid 6 may be turned in the directions indicated by the arrows β as shown in FIG. 7 in synchronism with the turning of the radiation source 12. Consequently, an additional mechanism is required to move the grid 6, and also an additional space for moving the grid 6 therein is required to allow the grid 6 to be turned in synchronism with the turning of the radiation source 12. Another problem is that the quality of the generated image of the breast 4 tends to be lowered because the positional relationship between the grid 6 and the radiation detector 2 varies as the grid 6 moves.